Asymmetric digital subscriber line (ADSL) technology has been introduced into the field of broadband networking, among other reasons, to overcome issues faced by traditional voice band technology. Such issues include, but are not limited to, bandwidth limitations. ADSL technology utilizes the infrastructure already in place in a public switched telephone network (PSTN), including copper loops, constructed of copper wires, between a customer premise and a central office. Advantageously, ADSL technology does not require replacement of network equipment such as routers, switches, firewalls and web servers, which are commonly used in today's paradigm for broadband access.
Unfortunately, in a telephone network, while electrical energy is transmitted across copper wires, a modulated signal also radiates energy onto adjacent copper wire loops that are located in the same cable bundle. This cross coupling of electromagnetic energy is referred to as crosstalk.
In a typical telephone network, multiple insulated copper wire pairs are bundled together into a cable called a cable binder. Adjacent systems within a cable binder that transmit or receive information in the same range of frequencies can create significant crosstalk interference. The crosstalk interference is attributed to crosstalk-induced signals combining with signals that were originally intended for transmission over the copper wire loop. The result is a slightly different shaped waveform than was originally transmitted, which is representative of data degradation.
Crosstalk can be categorized in one of two forms. Near end crosstalk, commonly referred to as NEXT, is the most significant because a high energy signal from an adjacent system can induce relatively significant crosstalk into the primary signal. In other words, NEXT essentially is a measure of the crosstalk noise that two devices used for communication purposes induce upon each other at the same end of the cable binder.
Another form of crosstalk is far end crosstalk, or FEXT. FEXT is typically measured by applying a test signal to a wire pair at one end of a channel and measuring the disturbance on other wire pairs at the far end. Therefore, FEXT essentially measures the crosstalk noise that communications equipment creates for devices it is communicating with at the opposite end of the wire pair used for data transmission. Typically, FEXT is less of an issue than NEXT since the far end interfering signal is attenuated as it traverses the copper wire loop.
In an environment, where Time Compressed Multiplex Integrated Service Digital Network (or TCM-ISDN) is deployed, the source of NEXT and FEXT noises are commonly referred to as TCM-ISDN interferers. The TCM-ISDN system performs alternatively upstream and downstream transmission of data during a period referred to as the TCM timing reference (TTR). In the first half of the TTR, an ISDN central office (ISDN-CO) transmits data to an ISDN remote terminal (ISDN-RT), while during the second half of the TTR period the ISDN-RW transmits data to the ISDN-CO. Consequently, an ADSL transceiver, connected to the wire loop via the CO end, receives NEXT noise from the ISDN-CO during the first half of the TTR, and FEXT noise from the ISDN-RT during the second half of the TTR. Conversely, an ADSL transceiver unit, connected to the wire loop via the ISDN-RT end, receives FEXT noise from the ISDN-CO during the first half of the TTR period, and NEXT noise from the ISDN-RT during the second half of the TTR period. The presence of ISDN NEXT and FEXT noises affects the performance of an ADSL system differently.
With the introduction of local loop use for both analog voice and digital data, Discrete MultiTone (DMT) line coding techniques were developed. DMT line coding techniques utilize multiple carriers that create sub-channels (i.e., 256 sub-channels for ADSL), wherein each sub-channel carries a portion of the total information for transmission. Each sub-channel is independently modulated, thereby accustoming DMT based ADSL modems to be considered multiple simultaneously running modems, operating in adjacent bands.
During initiation of an ADSL system which uses DMT line coding, DMT monitors line conditions and computes the bit carryino capacity of each sub-channel based upon the sub-channel's SNR. The system then accordingly assigns a number of bits to be carried by each sub-channel. If a sub-channel is experiencing external interference, such as, but not limited to, crosstalk a decision may be made either to exclude use of this sub-channel in favor of alternative sub-channels, or to vary its bit carrying capacity accordingly. Thus, DMT can dynamically adapt the bit rate to line conditions since each sub-channel carries a certain number of bits depending on its SNR, thereby automatically adjusting bit rate.
In a time duplex noise environmnent, such as, but not limited to, TCM-ISDN, an ADSL DMT system adapts its bit rate in accordance with the FEXT and NEXT noise levels. The overall rate of information transmission is dependent upon the number of DMT symbols transmitted, which, in turn, is affected differently by FEXT noise than by NEXT noise. Unfortunately, the number of symbols transmitted under the influence of NEXT noise is larger than the number of symbols affected by FEXT noise. Since typically NEXT noise is more powerful than FEXT noise, the overall bit rate of an ADSL system is essentially reduced due to a lower bit rate being used for the transmission of information during NEXT noise.